How to compare and replace (int) pointers?

Question

So i have this piece of code:

     int* get_lotto_draw() //Returns an array of six random lottery numbers 1-49
{
     int min = 1;
     int max = 49;
     int counter = 0;

     srand(time(NULL));

     int *arrayPointer = malloc(6 * sizeof(int));

     for(counter = 0; counter <= 5; counter++)
     {
                 arrayPointer[counter] = rand()%(max-min)+min;
     }  

     return arrayPointer;
}

This gives me 6 int* but sometimes these int* values can be the same. How can i compare each of them to eachother, so that if they are the same number, it will re-calculate on of the values that are equal ? Thanks for your time.

Could you explain what you are trying to do ? It seem to me that you are trying to compute a *random permutation*, but I'm not sure — hivert, Mar 23 '14 at 17:14
possible duplicate of [Unique random numbers in O(1)?](http://stackoverflow.com/questions/196017/unique-random-numbers-in-o1) — aschepler, Mar 23 '14 at 17:16
Im trying to create 6 random ints to simulate a lottery ticket, but each of these numbers must be unique, and if they are not i need to replace them or generate an entire new sequence. Im looking for the best way to sort through and compare each number as im unsure of how to do so. — hjalpmig, Mar 23 '14 at 17:19

LearningC · Accepted Answer · 2014-03-24T06:04:10.047

1

make a search in the array for same number before storing the number as,

for(counter = 0; counter <= 5; counter++)
     {
        int x1 = 1;
        while(x1)
        {

          int temp = rand()%(max-min)+min;
          for(i = 0; i < counter; i++)
          {
             if(arrayPointer[i] == temp)
             {
                break;
             }
          } 

          if(i == counter)
          {
            x1 = 0;
            arrayPointer[counter] = temp;
          }
        }
     }

edited Mar 24 '14 at 06:04

answered Mar 23 '14 at 17:23

LearningC

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19

Thanks! This solved the problem for me, I have tested multiple times and always get correct values. – hjalpmig Mar 23 '14 at 18:02

score 0 · Answer 2 · answered Mar 23 '14 at 17:53

The problem is that random-number generators don't know or care about history (at least, not in the sense that you do--software random number generators use recent results to generate future results). So, your choices are generally to...

Live with the possibility of duplicates. Obviously, that doesn't help you.
Review previous results and discard duplicates by repeating the draw until the value is unique. This has the potential to never end, if your random numbers get stuck in a cycle.
Enumerate the possible values, shuffle them, and pick the values off the list as needed. Obviously, that's not sensible if you have an enormous set of possible values.

In your case, I'd pick the third.

Create an array (int[]) big enough to hold one of every possible value (1-49).
Use a for loop to initialize each array value.
Use another for loop to swap each entry with a random index. If you swap a position with itself, that's fine.
Use the first few (6) elements of the array.

You can combine the second and third steps, if you want, but splitting them seems more readable and less prone to error to me.

How to compare and replace (int) pointers?

2 Answers2